Prepare: run E2E smoketests with GitHub actions (#23301)

- Port E2E testing scripts from cypress-ui-automation
- Move server to docker-compose, move E2E images to ecrpublic
* Integrate General channel renaming, fixes
- Add local automation-dashboard
- Add readme
- Add E2E smoketests
- Bump postgres to 12

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Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Saturnino Abril <saturnino.abril@gmail.com>
Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>

e2e-tests/.ci/.gitignore

@@ -0,0 +1,5 @@
+dashboard
+env
+!.env.cypress
+!.env.server
+!.env.dashboard